[patch] Exaile needs to handle vorbis embedded cover art better

[Christian Kampka]

Exaile needs to improve vorbis embedded covers in several ways:

First, the recommended tag is METADATA_BLOCK_PICTURE which is currently not supported. The COVERART tag is considered deprecated. See http://wiki.xiph.org/index.php/VorbisComment#METADATA_BLOCK_PICTURE for details.
Second, the TagCoverFetcher should not break the check loop if it encounters an empty tag, but continue looping until it finds a tag that actually contains some data.

[Christian Kampka (kampka)]

I have extended my previous patch so that exaile can now handle special FLAC covers as well.

[Christian Kampka (kampka)]

I have reworked my patch again, it contains a cleaner solution now.

[reacocard (reacocard)]

Can we rework the FlacCoverFetcher to just return those blocks as tags in xl/metadata/*, and then adjust TagCoverFetcher to pick up on them? I think that would be a cleaner solution, with the this one we run the risk of parsing the file twice if covers are not found - once in FlacCoverFetcher and once in TagCoverFetcher. metadata_block_picture at least should be trivial to move, and i can't imagine the flac-specific one will be hard either.

[Christian Kampka (kampka)]

Adjusted patch according to reaocards comments on IRC.

[reacocard (reacocard)]

committed r3382, thanks!